Even without a STAR or a runway, the box knows the elevation of the landing airport so will have a rough TOD point displayed - It just needs a start point (your cruise level) and an end point (landing elevation). When you enter the approach, it will be more exact. 3 times and a bit is a rough guide though. ie. 100nm at 30,000'. 50nm at 15,000'. Then just build in your slow down. 1 extra mile for a 10 knot slow down whilst conducting a continuous descent. All the best!